The present invention relates to the field of receivers for compatible AM stereo reception and, more particularly, to the prevention of increased signal degradation during periods of low S/N ratio.
In a stereophonic receiver for receiving an AM stereophonic signal of the form (L+R) cos (.omega..sub.c t+.phi.) where .phi. is arc tan [(L-R)/(1+L+R)], a correction factor proportional to cos .phi. is employed as a divider in order to restore the original and undistorted L and R signals. The cosine factor may be employed once or twice in various stages of the receiver, depending on the design of the receiver. A complete transmitting and receiving system utilizing the above recited compatible signal is fully described in a co-pending application, Ser. No. 674,703, assigned to the same assignee as is the present invention. This patent may be referred to for further details as to the transmitted signal, its characteristics and advantages. Another co-pending application also assigned to the same assignee is Ser. No. 837,258, pertaining to another stereo receiver utilizing the same signal. The receiver of the latter application will be referred to hereinbelow. As may be seen, the envelope of the signal contains only L+R or monophonic information, and no distortion due to the stereo information is produced in monophonic receivers.
As is known, in a typical received audio signal, very little of the contained energy is representative of the higher sound frequencies representative of information; e.g., the highest fundamental frequency played by a piccolo is only slightly higher than two kHz and higher harmonics of voice, instruments, etc., have little energy. Thus, when relatively high energy high frequencies are present in a demodulated signal, they are practically always due to noise or, in other words, the S/N ratio is very low. When such a noisy signal is processed normally in the cosine correction circuitry of a stereo receiver, division by the cosine correction factor further degrades the already poor signal. It is therefore advisable to reduce or eliminate division by the correction factor during periods of low S/N reception. Such periods may have a duration of as little as a fraction of a modulation cycle.